There have been known strain wave gearings provided with a cup-shaped or silk-hat-shaped flexible externally toothed gear. In a strain wave gearing, the flexible externally toothed gear thereof is set to be a reduced-speed-rotation output element, from which a reduced-speed rotation is transmitted to an output shaft, for example. While, in a strain wave gearing disclosed in Patent document 1, a rotation transmitted from an input shaft to the flexible externally toothed gear is reduced in speed and is outputted from the rigid internally toothed gear side.
Fastening between a flexible externally toothed gear and an output shaft, an input shaft or other shaft members is carried out in such a manner that a rigid boss formed in the flexible externally toothed gear is fixedly fastened in a coaxial manner to a shaft member with bolts being fastened. For example, the boss of a flexible externally toothed gear is sandwiched between a pressing plate and an input shaft, and in this state, is fixedly fastened in a coaxial manner to the input shaft by using bolts.